How sodium silicate furnaces work
Sodium silicate, or water glass, is made by melting silica sand with soda ash at about 1,300 to 1,450°C in a regenerative or recuperative tank furnace, typically 50 to 300 tonnes per day. The molten glass is tapped and cooled into lumps, then dissolved in water under pressure to make liquid silicate. Many plants fire furnace oil, coal gas or producer gas, others natural gas. The soda ash releases a large volume of CO₂ as it reacts, which adds to the flue gas, and sodium vapour and fine carryover leave with it as sodium carbonate and sulphate fume. Waste gas leaves the regenerators or recuperator at about 450 to 700°C. Older plants run on chimney draft; newer ones add a waste heat boiler or gas cooler, a bag filter or ESP, and an ID fan.
Where the ID fan sits
- Melting tank
- Regenerators or recuperator
- Waste heat boiler or gas cooler
- Bag filter or ESP
- ID fan
- Chimney
The fan sits after the dust collector where one is fitted, or at the chimney base on plants without gas cleaning, where it handles hot, dusty gas directly from the regenerators. With a waste heat boiler and bag filter, gas reaches the fan at about 180 to 250°C. Without them, it can be 400 to 550°C with significant sodium fume. Suction is typically 100 to 250 mmWC. Furnace pressure is held slightly positive.
Gas at the ID fan
| Gas temperature, normal | 180 to 250°C after cooling and filtration; 400 to 550°C without |
| Maximum or upset | Up to 600°C on uncleaned systems during regenerator upsets |
| Dust at the fan | Fine sodium carbonate and sulphate fume; heavy where there is no filter |
| Moisture and dew point | Moderate; dew point 50 to 60°C with gas or oil firing |
| Corrosive species | SO₂ and SO₃ from furnace oil or producer gas; alkali fume corrosive when damp |
| Gas composition | High CO₂ from soda ash decomposition, raising gas density and volume per tonne |
| Density at the design temperature | About 0.71 kg/m³ at 230°C, 59% of standard air. Pressure developed and power drawn scale with density, so the fan is selected at this condition and the motor is checked at the coldest operating or start-up condition. |
ID Fan selection for sodium silicate furnaces
| Blade form | Backward inclined or radial tip, chosen for the deposit risk |
| Material of construction | Mild steel or boiler plate after a filter; heat-resisting steel on uncleaned hot gas above about 400°C |
| Drive and arrangement | Direct coupled or belt driven, arrangement 8 or 9, with sliding supports on hot duties |
| Bearings and cooling | Cooling disc and heat shield; water-cooled bearings on hot uncleaned duties |
| Flow control | Variable speed drive on furnace pressure |
| Also specified | Cleaning doors, chimney bypass and insulation above the acid dew point |
What this ID fan has to cope with
Sodium fume deposits
Sodium silicate furnaces evaporate more alkali than most glass furnaces because the melt is rich in soda. The fume condenses as sticky sodium carbonate and sulphate that coats regenerator checkers, ducts and fan blades. On a fan without a filter ahead of it, this builds quickly and causes imbalance. A filter upstream, or a radial tip blade with cleaning doors, keeps it manageable.
Underestimating gas volume
Soda ash releases a large amount of CO₂ as it reacts, which adds substantially to the combustion gas. A fan sized only from fuel consumption, as for a boiler, falls short at full pull. The batch gas must be added to the combustion gas when calculating the fan duty.
Acid corrosion with fuel oil
Plants firing furnace oil or producer gas carry SO₂ and SO₃, and the alkali fume reacts to form sulphates that are hygroscopic. At shutdowns the deposits absorb moisture and become corrosive to the casing and impeller. Keeping the fan above the acid dew point in service, full insulation, and prompt cleaning and drying after stops extend its life.
Energy and control
Sodium silicate furnaces run continuously, but pull varies with demand for liquid silicate, and many plants run below capacity for long periods. A variable speed ID fan follows pull and the gradual fouling of regenerators without throttling. Holding the correct furnace pressure also cuts air ingress, which matters on older furnaces with worn ports and doghouses.
ID fan and FD fan in sodium silicate furnaces
| ID fan | FD fan | |
|---|---|---|
| Position | After the filter or at the chimney base | Before the regenerators or recuperator |
| What it handles | CO₂-rich waste gas with sodium fume | Ambient filtered air |
| Temperature at the fan | 180 to 250°C cleaned; 400 to 550°C uncleaned | Ambient |
| Main design concern | Alkali deposits and gas volume | Pressure margin for checker fouling |
| Control | Furnace pressure | Air flow ratioed to fuel |
See also the Forced Draft (FD) Fan for sodium silicate furnaces.
Questions buyers ask
Why does a sodium silicate furnace produce so much flue gas?
Besides the combustion gas, the soda ash in the batch decomposes and releases CO₂, which leaves with the waste gas. In a sodium silicate furnace, with its high soda content, this batch gas is a significant addition. The ID fan duty must include it, along with any dilution air.
What causes deposits in a sodium silicate furnace ID fan?
The melt is rich in soda, and sodium evaporates from it as fume. The fume condenses as sticky sodium carbonate and sulphate on cooler surfaces, including the fan impeller. A bag filter or ESP ahead of the fan removes most of it; without one, a deposit-tolerant blade and regular cleaning are needed.
Can a sodium silicate furnace run on chimney draft?
Many older furnaces do, with a tall chimney at the end of the regenerator flues. Once a waste heat boiler, bag filter or ESP is added, its resistance requires an ID fan. A fan also gives steadier furnace pressure than a chimney, which helps on furnaces whose pull and firing rate vary with demand for liquid silicate.
Information needed to quote an ID fan for sodium silicate furnaces
- Duty point. Gas volume in m³/h or Am³/h at the fan inlet and the static pressure the fan must develop, with the margin you want on each.
- Gas condition. Normal and maximum gas temperature at the fan, dust loading after the collector, moisture and any acid gases such as SO₂ or HCl.
- System. Boiler or furnace capacity and fuel, collector type and its pressure drop, and the existing fan curve or nameplate if this is a replacement.
- Mechanical. Drive arrangement, flow control (damper, inlet vanes or variable speed drive), rotation and discharge orientation, and a layout drawing or site photograph.
- Batch and pull. Silica to soda ash ratio, pull rate and fuel type.
- Gas cleaning. Whether a waste heat boiler, bag filter or ESP is fitted or planned, and gas temperature at each.
Built and tested in Chennai
Adroit Fantech Engineers has designed and manufactured industrial fans at Ayanambakkam, Chennai since 2004. Each impeller, casing and shaft is built for the gas and temperature it will see, performance tested as per IS 4894 or BS 848 Part I, and balanced to grade G 6.3 or G 2.5 as per ISO 21940-11, with the records supplied with the fan. We also replace rotors and retrofit existing fans of any make.